(1) Field of the invention
This invention relates to breathing apparatus, and more particularly to positive pressure breathing apparatus.
In positive pressure breathing apparatus a positive pressure greater than the ambient pressure is maintained within a protective device such as a face mask or a hood surrounding the respiratory passages of a wearer. The use of positive pressure has the particular advantage that, if there is any leakage, the leakage will all be outward from the protective device thus providing a high degree of protection against the ingress of noxious fumes into the gas which the wearer of the apparatus is breathing.
Positive pressure filter respirators are one form of positive pressure breathing apparatus in which the positive pressure is provided by means of an air mover such as a blower or a fan which delivers air through a filter to the interior of a face mask, helmet or hood. The air mover, which may be a battery-driven electrical device, removes from the wearer the need to overcome the resistance of the filter and so reduces the breathing effort required.
In such conventional positive pressure respirators the positive pressure is maintained by means of a high flow of air, at least 120 liters per minute but preferably of the order of 160 liters per minute. Most of this air is wasted since the 20-40 liters per minute of air which is required for breathing is required during the inhale part of the cycle only, but the air mover continues to supply air during the remainder of the breathing cycle. Whilst in some cases the high flow of air is beneficial in terms of cooling the wearer, it is wasteful in terms of filter life and battery life.
Positive pressure filter respirators for use in highly toxic environments comprise an inner or orinasal mask within an outer protective member, which may be a hood, a helmet or a mask, a barrier layer of filtered gas being maintained in the outer protective member around the inner or orinasal mask.
(2) Description of the prior art
In U.K. Patent Specification No. 2,032,284 A it is proposed to improve the filter life in breathing apparatus of this kind generally by detecting exhalation by the wearer and at least reducing flow of air through the filter during at least part of each exhale part of the breathing cycle of the wearer. It is disclosed that, in breathing apparatus having no inner or orinasal mask, the air mover or pump may be stopped, but it is preferred that the speed of the air mover is reduced. In breathing apparatus having an inner or orinasal mask and an outer mask, the speed of the air mover is regulated in consequence of the build up in pressure in the outer mask following closure of inhale valves in the inner mask during exhalation. This build up is detected by a detector in the outer mask operating in response to the pressure difference between the outer mask and the ambient atmosphere. Because the exhale valve in the inner mask opens directly to atmosphere, the build up in pressure in the outer mask occurs as a result of the continued running of the fan after exhalation commences, and there is consequently a slow response of the detector to the commencement of exhalation and the improvement in the filter life and battery life is significantly less than it might be.
In U.K. Patent Specification No. 2,141,348 A there is disclosed breathing apparatus which is a positive pressure filter respirator having inner and outer masks. The life of the filter in this apparatus is extended by disconnecting the pump means or fan from the power means in response to detection of the pressure of air between the pump means and the filter means by a pressure sensor. Again there is a slow response to the commencement of exhalation because the pressure which is sensed is dependent on the continued running of the pump means after the inhale valves in the inner mask close. There is therefore delay before the pump means or fan is disabled. Also, considerable effort by the wearer is required at the commencement of inhalation in order to reduce the pressure within both the inner and outer masks below atmospheric before the pressure sensor located between the pump means and the filter will reconnect the power means to the pump means or fan. There is therefore considerable risk of noxious gases leaking into the outer mask during this early portion of the inhale part of the breathing cycle.